The present invention relates to a new and improved construction of a circuit arrangement for generating a sinusoidal synchronisation voltage for the firing of thyristors in a current regulation circuit of a drive for an electric motor.
It is known for thyristor drives to derive the synchronisation voltage for the firing of the thyristors from the same power supply network from which there is removed the drive current. However, this is not possible when working with high-output drives which are employed, for instance, for different types of military equipment and which are powered by small 400 Hz generators of the type used in the aircraft industry, at least for the following reasons:
In the observed equipment the network frequency markedly varies, especially if the generator is provided with a flywheel or driven plate acting as an energy storage. The delivery of the drive output and the take-up of the brake load by the generator can cause fluctuations of the network frequency between about 350 Hz and 450 Hz. PA0 Upon commutation breakdown of the voltage at the thyristors the product of shift and time duration for a voltage breakdown, among other things, is proportional to the frequency and the inductance of the supply network as well as to the load current. Thus, the product of the shift and time duration for the considered supply network of about 400 Hz is appreciably greater than with a network working at 50 Hz and, additionally, is markedly dependent upon load and current, respectively. The fundamental wave or waveform of the undistorted network voltage, because of the commutation breakdown, is superimposed upon a non-sinusoidal waveform, whose fundamental wave is phase shifted in relation to the undistorted network voltage as a function of the current, in other words the firing angle. The resultant fundamental waveform of the network voltage is shifted in relation to the fundamental waveform of the undisturbed network voltage by a variable phase angle, which can amount up to .+-.20.degree.. Under these circumstances the known simple filter circuits for the extraction of a sinusoidal synchronisation voltage from the resultant distorted network voltage are incapable of working satisfactorily. In the description to follow there is to be understood under the expression "distortion" a disturbance of the network voltage which essentially affects the sinusoidal shape and/or the phase.
Because in the considered equipment the peak load essentially consists of thyristor bridge circuits loaded by the electric motor and the considered power supply network containing the aforementioned small generator and the infeed lines possess a greater inductance than conventional networks, such as for instance the power supply network of a city or municipality which powers conventional drives, there arises during cut-on and during load change a much more pronounced fluctuation of the network voltage in the supply network of the considered equipment than in conventional networks. In the description to follow there is to be understood under the term "fluctuations" a disturbance of the network voltage which essentially affects the amplitude.